The area of medicine, and particularly that field of human medicine devoted to individual and public health, has long sought a non-toxic compound that could be administered for effective parasite control.
As is known, there are three general types of parasites, namely, intracellular, those that occur in tissues, and those that live in the blood stream. Malaria is a principal example of a type of parasite that lives in the blood stream but it should be noted that malaria can also live in cells.
Intracellular protozoa have been observed to enter host cells in subtle ways, escape the cells digestive processes, and instead have the cell nourishing the parasite, as reported by Professor William Trager in an article entitled "Some Aspects of Intracellular Parasitism" appearing in Science, Volume 183, pages 269-273, Jan. 25, 1974.
As observed by Professor Trager, the environmental requirements and nutritional needs of an intracellular parasite are sufficed only by intact living host cells with the types of intracellular organisms varying over a great range. With respect to particular parasites, it has been observed that some intracellular protozoa appear to enter a host cell by the host cell engulfing the parasite by a phagocytic process, and included in this category are the hemoflagellates of the genus Leishmania, for example, which are responsible for a variety of diseases.
The Leishmanial parasites, while entering a host cell without evident activity of the parasite, nevertheless interacts with the cell in a biochemical manner with the cell forming a membrane about the parasite that nourishes the parasite rather than destroying it.
Other intracellular parasites, for example the Toxoplasma gondii, enter host cells by their own activity rather than that of the host cell. This parasite must have a living host cell and can grow and multiply only intracellularly with, again, the host cell being made to nourish the parasite.
Still other intracellular parasites, namely the Microsporida, do not have a membrane formed therearound, but instead form resistent spores with a tubule that extends from the spore and is attached to a host cell so as to be directly in contact with the cytoplasm. Thus, it is not always necessary that the host cell have a membrane around the parasite, as shown again, by example, by erythrocytic malaria, the sporoplasm of Nosema, or the erythrocytic parasite Babesia.
In any event, however, erythrocytic parasites feed by endocytosis and it has been speculated that this may be the general method of feeding by protozoa parasitic in other kinds of cells. By endocytosis feeding, the host cell, of course, at least helps in nourishing the parasite. With respect to the erythrocytic stages of malaria, it is known that the malaria parasite developing within an intact host has relatively few nutritional requirements that must be satisfied from outside the host cell since the host cell provides glucose, a few amino acids, fatty acids and a few vitamins. In fact, the host cell sometimes must modify at least one vitamin before it can be utilized by the parasite. This is true of the malaria parasite P. lophurae which cannot synthesize its own coenzyme A from pantothenic acid and depends upon modification by the host cell. Therefore, the intercellular parasite often finds the host cell to be a true and hospitable host, and the parasite not only exploits the nutrients already available in the cell, but induces the cell to actively assist in nutrition of the parasite.
Since at least some parasites depend upon host cells for nourishment, it is apparent that if the source of the energy to the cell utilized by the parasite can be effectively cut off, then the parasite can be destroyed.
A few years ago, this inventor invented a novel sulfur-containing compound and method for the preparation of the same and U.S. Pat. No. 3,243,425 was issued to me on Mar. 29, 1966, the invention being assigned to Purdue Research Foundation. The sulfur compounds of that invention are based upon the replacement of an oxygen atom in a sugar molecule by a sulfur atom, and, more particularly, are based upon the replacement of the ring oxygen of the sugar by the sulfur atom and oxidized forms of the sulfur atom and thus may be described as thiosugars.
While the compounds described in my U.S. Pat. No. 3,243,425 were recognized to be of both chemical and biochemical interest as sugar analogs, the main recognized use of the compounds was primarily in the preparation of resins by reaction with a diiocyanate or other polyisocyanates, with usefullness as radiation absorbers and as chain terminators in free radical polymerizations being mentioned. More recently, I have found that 5-Thio-D-glucose is also useful as a tumor cell growth restricting compound and as a weight control compound. Patent applications directed to such compound uses have been made the subject matter of U.S. patent applications filed by me on Feb. 14, 1974 and given Ser. Nos. 442,448 and 442,447, respectively. It has remained until now, however, to find and develop usefulness for 5-Thio-D-glucose for parasite control.